This invention relates to systems for changing the speed of a wound rotor motor and recovering the slip energy thereof, the former being achieved by removing controlled amounts of power from the rotor of the wound rotor motor and the latter by returning the removed power to the power source for the wound rotor motor.
It is well known that the speed of a wound rotor motor can be adjusted by changing the power flow through the rotor circuit of the motor. With early wound rotor motor speed controls the removed power was dissipated in adjustable resistor banks or in large liquid-cooled rheostats, an obviously wasteful use of the power. Subsequently, slip energy recovery (SER) systems were developed making it possible to recover the previously wasted power inverting it back to 60 Hz AC power and returning it to the power source for the motor through a voltage matching transformer.
Slip energy recovery systems have been employed in North America for about ten years. Prior art systems presently marketed share a common problem, particularly as the system becomes a large part of the total installed electrical load. Thus, due to the characteristics of the line-commutated inverter employed in the SER system, the reactive current drawn from the supply when the motor is operating at full speed and full load is equal to the full-load motor current, resulting in a total KVA draw from the supply that is almost double the normal full-load value. Fortunately this can be reduced to normal values by connecting power factor correction capacitors across the system input, these being equal in rating to the full KVA rating of the motor.
While the use of power factor correction capacitors reduces the full speed KVA to more normal proportions, it introduces other problems with potentially greater impact on the system. These include:
(a) A leading power factor at speeds below 75% resulting in supply system instabilities, particularly in the presence of emergency diesel-driven standby power generators.
(b) A large capacitive standby current when the motor is not actually running.
(c) Harmonic pollution of the supply caused by resonance of the large capacitive component with the supply impedance (inductive reactance) with resulting higher voltages, system overloads, capacitor fuse operation and magnetic saturation.
By way of elaborating on the foregoing, if the SER system becomes a large part of the total electrical load, which may be, for example, a complete pump house having lights, heaters, fixed speed drives etc. in addition to SER systems, the AC transformer supplying power to the pump house will have a large inductance that will resonate with the large power factor correction capacitors at a harmonic (the 5th) that is the same as that of the inverter-rectifier subsystem of the SER system. Also, if the AC supply should be lost, power then may be supplied on an emergency basis from a diesel generator. At this point the wound rotor motor is not operating. When the diesel motor gets up to speed, the correct field is supplied to the generator driven by it to give the required voltage, and when this generator is connected to the system the result is a leading current to supply the large power factor correction capacitors. This creates an unstable generator because the field of the generator becomes self-exciting. The same thing can happen if the system is being supplied from a diesel generator and the speed of the wound rotor motor is reduced, i.e., depending on the extent of the speed reduction, a leading power factor can result.
A number of methods have been employed to limit the effect of the large power factor correction capacitors employed with SER systems including switching banks of capacitors onto the system as the motor load is increased and switching them off again as the load decreases. This solution sharply increases the cost of the overall system, decreases its reliability due to the increased mechanical stress on the switching contactors and generates undesirable switching transients on the system supply.